Wireline communication systems often support more than one communication technology. For example, a conventional wireline communication system may support several different Ethernet technologies such as 10BASE-T (10 Mbit/s), 100BASE-T (100 Mbit/s), Gigabit Ethernet (1 Gbit/s), 10 Gigabit Ethernet (10 Gbit/s) and/or 100 Gigabit Ethernet (100 Gbit/s). In another example, both DSL (digital subscriber line) and E-carrier/T-carrier technology (E1/T1) may be supported. Line driver circuitry included in such systems must be capable of supporting the output operating conditions specified by each supported technology such as output voltage swing, output current, termination impedance, gain, bandwidth, etc. Otherwise, signal quality degrades significantly.
Different wireline communication technologies often have widely dissimilar output operating conditions. Some types of conventional line driver circuits accommodate the worst-case output operating condition, sacrificing power, signal quality and/or performance when operating under less extreme conditions. For example, a line driver circuit may be designed to operate at the greatest output voltage swing supported by the system. However, power is wasted when the line driver operates in accordance with a different wireline communication technology having a lower output voltage swing condition. Another conventional approach involves changing the external devices coupled to the line driver circuitry that affect the output operating conditions such as transformer winding ratios, external termination impedances, etc. This way, external components may be switched out or externally re-configured to implement a different wireline communication technology. This approach increases the area and cost of the system and limits configurability.
Another issue facing wireline communication systems is the increasing number of output ports included in such systems. This is particularly applicable for Ethernet-based systems, where four line drivers are typically employed per Ethernet port. Ethernet line drivers conventionally have a negligible output impedance and a termination impedance matched to the line impedance for reducing interference. The line driver supply voltage is typically at least twice the output signal swing of the driver to account for the voltage drop at the termination impedance. Thus, at least half of the power consumed by the line driver is wasted. Heating also becomes problematic as the number of ports (and thus line driver circuits) increases. Moreover, it becomes extremely difficult to fabricate Ethernet line drivers using advanced semiconductor technologies because high supply voltage requirements typically exceed technology voltage limits. Device breakdown occurs if these voltage limits are exceeded.